When millions of Americans abandoned smoking in the 1980s, many health experts and social scientists thought they had tobacco on the run. But in the '90s, progress began to slow: From 1990 to 2003, according to federal figures, only 3 percent of Americans gave up their cigarettes.

So social scientists turned to a new quarry: understanding the mind of the smoker.

What they have found has proved more controversial than most researchers expected: Smokers are more depressed and suffer a higher rate of anxiety disorders and other psychological maladies. At the same time, nicotine may provide a mental boost that helps them cope. These findings help explain why some people won't quit, experts say.

"We thought understanding the smoker's mind would help us end tobacco use," says Gerald Markle, professor of sociology at Western Michigan University and author of Cigarettes: The Battle Over Smoking . "But, in some respects, we've raised as many new questions as we've answered."

Recent studies show smokers are 4.7 times more likely than the population at large to suffer from major depression. Dozens of other surveys reveal cigarette users are more liable to struggle with anxiety disorders, schizophrenia, attention-deficit hyperactivity disorder, alcoholism and drug use.

Smokers consistently demonstrate higher-than-average levels of neuroticism and high-risk behaviors, and show poorer impulse control than nonsmokers.

These and other smoker profiles have influenced public health campaigns, contributing to the development of therapies that treat depression alongside cigarette addiction. But the studies are starting to worry smokers: Some private and public employers are using the findings to justify employment decisions.

In January, two researchers hired by the Naval Postgraduate School in Monterey, Calif., to profile the smoking habits of recruits suggested possibly excluding longtime smokers because they are at higher risk of expulsion for behavioral issues.

"Smoking suggests other types of problematic behavior," says Eli Flyer, one of the study's authors and a former senior analyst for the Department of Defense.

The Navy has not announced plans to revise enlistment policies based on the findings. But scientists involved in the studies say such interpretations of their research miss the point. Rather than suggesting that smokers should be denied opportunities such as employment, they argue the research suggests that smoking addiction is treatable.

"Smokers are socially isolated and so less likely to search out help," says David Gilbert, a professor at Southern Illinois University who examines how nicotine affects the brain. "But these studies suggest that better treatments are out there."

One of the treatments under discussion is using nicotine to combat depression.

Experiments conducted by Gilbert demonstrate that smokers and ex-smokers focus better on positive thoughts and images after receiving nicotine from a patch, suggesting the drug may be useful in treating the lethargy and pessimism associated with depression.